GB2282866A - Shock absorbing arrester - Google Patents

Abstract

An in-line shock absorbing arrester adapted to be connected between two lengths of safety line, comprises a telescoping piston 3 and cylinder 5 assembly in which the movement of the piston in the cylinder, such as occurs in the event of a shock, causes the successive rupturing of a plurality of plugs 7, thereby to retard the movement of the piston. <IMAGE>

Description

SHOCK ABSORBING ARRESTER This invention relates to a shock absorbing or energy absorbing arrester, and in particular, it relates to an arrester, having shock absorbing properties, the arrester being adapted to be connected, in a safety line, lifeline, cable, lanyard, shock cord, or the like which is adapted to support load, including shock load, in tension. Such a shock absorbing arrester is intended to have an ultimate tensile strength greater than the safety line lengths between which it is connected, and it is intended to provide some measure of shock absorption in order to cushion shock occasioned at some predetermined level of tensile loading which is well below that which would cause rupture of the safety line.

Such a shock absorbing arrester may be employed for example between lengths of a safety line horizontally strung from a fixed structure in a fall arrest system provided for a worker when the worker is required to attend to tasks at elevated height above ground and requires protection in the event of a fall. The line is secured at each of its ends to an anchor which in turn will be secured to a strongpoint.

The strongpoint may be a strong steel structure in which case the anchor may be a strong steel element such as a ring securely bolted to the structure. This presents no problems, in contrast to the case where the strongpoint is a non metallic structure e.g. of brick or concrete. In this case, the anchor is often in the form of an expansion device part of which is entered in a hole drilled or formed in the non metallic strongpoint and the device has means whereby the inserted part may be expanded so as to hold to the walls of the strongpoint hole. Such an anchorage with a non-metallic strongpoint has limited strength and it is unsafe to assume that it can withstand a shock load much in excess of say 10 KN.

The purpose of the in-line energy absorbing arrester is to limit the maximum load which a safety line, for example a horizontally strung line, is liable to sustain. In the case of a safety system of the horizontally strung line type, several workers may be attached via lanyards to the horizontal line and it is known to incorporate in such lanyards, shock absorbing devices which permit the lanyard length to be extended under shock loading, such extension being accomplished by means which cushion the shock transmitted to the worker. Such a device is usually of the folded tear web type.

In the event of the fall of one worker, his lanyard absorber may ensure that the fall-arrest load profile does not at any instant exceed a predetermined maximum, say 6 KN. The computer programme used in a selection of design parameters for a given installation shows that a 6 KN load, transmitted to the horizontal line via the lanyard, can translate into a tensile load of twice that value on the horizontal line.

So the fall of one worker can impose a tensile load of say 12 KN on the horizontal line. In the event of the fall of two or more workers simultaneously, the load on the horizontal line (in the absence of an in-line shock absorbing arrester) can be prohibitive. The fall of four workers, e.g. due to collapse of a walkway, could cause such load to reach say, 48 KN, i.e. nearly 5 tons. Many non metallic building structures will not hold the horizontal line anchorages under such a load. The in-line arrester which is the subject of this invention has for one object to prevent loading transmitted from the horizontal line to its anchorage from exceeding safe limits. Indeed, one embodiment of the arrester described below is intended to reduce shock load transmitted to an anchorage by at least 50%, and preferably by at least 75 % so that only at most about 25 % of the shock load is transferred to the wall anchor fast with the building structure, or the like.

A safety lanyard may connect a workers body harness to a horizontally strung safety line by means of a coupling sleeve, or slider, which is slidable along the safety line, and the present invention is concerned to provide an arrester which has a cross-sectional size little larger than that of the safety line, so that the incorporation of the arrester in the line presents little impediment to the passage slidingly along the line, of a sleeve-like coupling, or slider, attached to a safety lanyard which in turn, is attached to a worker's body harness.

Broadly stated the present invention provides an in-line shock absorbing arrester adapted to be connected between two lengths of safety line, the arrester comprising a telescoping piston and cylinder assembly in which the movement of the piston in the cylinder, such as occurs in the event of a shock, causes the successive rupturing of a plurality of plugs, thereby to retard the movement of the piston. Such an arrester will be connected between two sections of safety line or the like with the piston secured to one section and the cylinder to the other.

Preferably also there are means for defining the maximum stroke permitted to the piston so that it is arrested at one end of its stroke and cannot be withdrawn through one end of the cylinder. To this end, the piston has a head which is a sliding fit in the bore of the cylinder and the piston head is carried on a piston rod which is of smaller diameter than the bore and this piston rod is a sliding fit in a bearing of reduced diameter at one end of the cylinder and the piston head is sufficiently large that it cannot pass the bearing and abuts a stop shoulder of the cylinder adjacent the bearing to arrest the stroke of the piston in the direction of movement towards the stop shoulder.

The wall of the cylinder has at various locations and at various distances from the bearing a series of holes and conveniently these holes are in one or more rows along the cylinder. In each hole, press fitted or screwed therein, is a plug, each plug extending into the bore of the cylinder. Accordingly, as the piston head moves, as a sliding fit, along the bore of the cylinder, it abuts the plugs, and breaks them.

If the plugs are in a row, or otherwise progressively spaced from the stop shoulder, they will be successively ruptured or sheared or otherwise broken as the piston moves towards the stop shoulder of the cylinder.

The plugs may be of a material which is markedly less tough than the material, of which the piston and cylinder are composed. Or preferably the plugs are sized and or shaped to break under a specified predetermined load. The plugs may be typically of aluminium brass or similar soft metal, although they may be of a synthetic plastics material such as polyamide.

One particular embodiment of arrester according to the invention is designed so that it can be installed between two sections of a horizontally strung safety cable without interfering with the free movement of the coupling component (slider) along the cable. The tubular body of the slider, which passes freely over the head portions of the supporting brackets of the horizontal line can also pass freely over the in-line absorber. However this "in-line" arrester can be installed at the end of the horizontally strung line, i.e. between an extremity of the horizontal cable and its anchorage point on the fixed structure.

A preferred embodiment has many pairs of plugs in rows, (the plugs of each pair being diametrically opposed) and a further plug (which must also break) which is for the purpose of preventing rotation of the piston during screwing of a horizontal line end fitting into the arrester cylinder. So if and purely for example, there are nine plugs in all and if each plug is designed to yield under a load of say, about 5 KN so the arrester is capable of absorbing loads up to 45 KN.

In an alternative arrangement, the plugs are disposed on a curve which extends helically around the cylinder, and are so disposed that successive plugs overlap one another in the axial direction. By this arrangement as the piston moves along the cylinder it continuously ruptures successive plugs, there being no intervals during which the piston can accelerate between plug ruptures. By these means a substantially more constant retarding effect is obtained. A similar effect can be acheived by providing extra rows of plugs the respective rows being offset radially of the cylinder, at say 90" between the rows, so that the piston head is substantially constantly engaged with one or more plugs as it moves along the cylinder.

The plugs may be simply force-fitted into holes in the cylinder.

But that form may be unsatisfactory for all purposes, as the plugs may be simply forced out of the cylinder. In another form, the plugs are given an external thread and they screw into screw-threaded bores in the cylinder wall. When the load on a pair of plugs exceeds a predetermined value the plugs rupture at locations where the threads emerge from the cylinder wall.

Such threaded plugs may be of stainless steel. Other materials possibly suitable are brass and aluminium. Instead of using plugs the absorbing elements can be formed by metal strips which extend transversely through the piston and project from it at diametrically opposed positions.

Another common form of in-line shock absorbing arrester is often employed in the lanyard which interconnects the worker's body harness with the safety line. This is often of the so called tear web type which employs folds of webbing, the folds being secured, as folds, by means such as stitching which is intended to rupture in the event of a shock such as occurs during a fall, the webbing becoming progressively unfolded until it can unfold no further, when it is adapted to arrest the fall and to support the fallen body. In such a system, the progressive rupture of the web fold securing means, as the webbing unfolds, damps the shock of the fall. The in-line absorber provided on a horizontally strung safety line for example, does not obviate the need for a separate shock-absorber in the worker's lanyard. The purpose of that shock-absorber is to keep the maximum fall-arrest load which can be sustained by the worker to a value within the limits prescribed by recognised standard specifications. However the shock absorbing arrester according to the present invention could be employed in a workers lanyard instead of or in addition to the shock absorbing device (typically of the folded web type) usually incorporated therein.

By way of example, one embodiment of the present invention is shown in transverse cross-section in the accompanying drawing.

The in-line shock absorbing arrester is generally designated 20 and is shown in the drawing connected between two sections la and lb of a horizontally strung safety cable which will be securely attached at elevated height to a fixture such as a building structure.

Secured to the horizontally strung safety cable section 1 a is a sleeve 2 whose end at 2a is given a reduced taper so as to provide minimal impediment to movement along the safety cable of a slider to which will be attached a worker's safety lanyard. At its end remote from the tapered end at 2a, the sleeve 2 has a socket 2b which receives the end of a piston rod 3a of a piston generally designated 3. The end of the piston rod 3a is secured by a pin 12 in the socket 2b of the sleeve 2.

Secured to the second horizontally strung safety cable section lb is a sleeve 4 which has a tapered end at 4a serving the same function as the tapered end 2a of the sleeve 2. At its end remote from the taper 4a the sleeve 4 is formed with an externally threaded stud 4b. The externally threaded stud 4b, is threaded into an internally threaded recess 5a of a cylinder 5.

The piston 3 is movable in the cylinder 5 against the resistance of shock absorbing means and this assembly constitutes the essential part of the shock absorbing arrester according to the invention and which is generally designated 20. The piston 3 has the rod 3a and at one end of this is a piston head 3b which is received as a sliding fit in a bore 5b of the cylinder 5. The wall of the cylinder 5 has a hole which can be aligned with a transverse hole in the piston head and a plug 6 is a press fit in these holes when aligned.

The piston 3 is inserted into the cylinder 5 (from its right hand end, as appearing on the drawing, with the piston rod 3a being fed in first), this location of the piston being accomplished prior to the insertion of the threaded stud 4b. The piston rod 3a is a sliding fit in a bearing Sc which is of a diameter smaller than that of the cylinder bore and adjacent the bearing Sc, the bore terminates in a stop shoulder 5d where this change of diameter occurs. The piston head 3b abuts this stop shoulder, when moving to the left in the drawing, so that the shoulder Sd constitutes a limit stop.

Between the pin 6 and the stop shoulder 5d, the wall of the cylinder is punctured by a series of holes, each occupied by a plug 7 which may be press fitted or threaded into its hole. These plugs 7 extend into the bore of the cylinder and movement of the piston head towards the stop shoulder can only be accompanied by rupturing the plug 6 and the plugs 7.

In the version illustrated, there are opposite rows of plugs 7 and the plugs in each row are progressively distanced from the stop shoulder Sd. As shown there are four plugs in each row but there may be many more for example twelve. As shown the piston has only one head for rupturing the rupture plugs 7 but the piston could have a number of heads on a common piston rod. However the or each piston head is arranged to rupture successive plugs as the piston is withdrawn from the cylinder.

The piston 3 will be withdrawn from the cylinder 5 (moving to the left of the drawing) in the event that the sections la and 1b of the horizontally strung safety line, of which they form a part, are subjected to shock loading. fhe withdrawal of the piston will cause successive rupturing of the plugs 6 and 7 and this rupturing will dissipate energy and provide shock absorption.

In an alternative arrangement which is not shown in the drawing, the holes for the plugs are disposed on a curve which extends helically around the cylinder 5, and these holes are so disposed that successive plugs placed in the holes overlap one another in the axial direction. By this arrangement as the piston moves along the cylinder it continuously ruptures successive plugs, there being no intervals during which the piston can accelerate between plug ruptures. By these means a substantially more constant retarding effect is obtained. A similar effect can be acheived by providing extra rows of plugs in holes as indicated at 17 in the drawing, these plugs at 17 being disposed overlapping or between the plugs 7 axially along the the cylinder in the region traversed by the piston head during the stroke of the piston In the embodiment of arrester illustrated the cylinder has the same or nearly the same thickness as the sleeves 2 and 4 so that a slider running along the safety lanyard can pass over the sleeves 2 and 4 and over the arrester 30 connected in line therebetween. There is therefore a design restraint caused by this particular functional requirement, and when the arrester is so used its external diameter is limited and must not be overlarge. When there is no such limit applicable to the diameter of the arrester, the arrangement of the cylinder with its plugs 7 may be re-designed so that the plugs 7 are adapted to bend, rather than break, as the piston head moves toward the stop shoulder.

Claims (10)

1. An in-line shock absorbing arrester adapted to be connected between two lengths of safety line, the arrester comprising a telescoping piston and cylinder assembly in which the movement of the piston in the cylinder, such as occurs in the event of a shock, causes the successive rupturing of a plurality of plugs, thereby to retard the movement of the piston.

2. An arrester according to claim 1 and including means for defining the maximum stroke permitted to the piston so that it is arrested at one end of its stroke and cannot be withdrawn through one end of the cylinder.

3. An arrester according to either of claims 1 or 2 and wherein the piston has a head which is a sliding fit in the bore of the cylinder, the piston head being carried on a piston rod which is of smaller diameter than the bore and this piston rod being a sliding fit in a bearing of reduced diameter at one end of the cylinder, and wherein the piston head is sufficiently large that it cannot pass the bearing and abuts a stop shoulder of the cylinder adjacent the bearing to arrest the stroke of the piston in the direction of movement of the head towards the stop shoulder.

4. An arrester according to any one of claims 1 to 3 and wherein the wall of the cylinder has at various locations over the stroke of the piston, a series of holes, there being a plug press fitted or screwed into each hole, with a part of each plug extending into the bore of the cylinder.

5. An arrester according to any one of claims 1 to 4 and wherein the plugs are progressively placed axially of the cylinder so that as the piston moves along the bore of the cylinder, it abuts the plugs, and breaks them successively.

6. An arrester according to claim 4 and wherein the plugs are so disposed that as the piston moves along the cylinder it is continuously in engagement with one or more plugs, so that, without such interval as might permit acceleration of the piston, successive plugs are always being sheared or otherwise broken as the piston moves in its stroke along the cylinder with the piston being constantly retarded.

7. An arrester according to claim 1 and wherein the plugs are disposed on a curve which extends helically around the cylinder, and the plugs are so disposed that successive plugs overlap one another in the axial direction, whereby as the piston moves during its styroke along the cylinder, it continuously ruptures successive plugs, there being no intervals during which the piston can accelerate between plug ruptures.

8. An arrester according to any preceding claim and wherein the plugs have an external thread and are screwed into screw-threaded bores in the cylinder wall.

9. An arrester according to claim 1 and wherein the said threaded plugs are of stainless steel, brass or aluminium.

10. An arrester according to claim 1 and substantially as hereinbefore described with reference to the accompanying drawing.